fix(functions): add missing Optional import in string.py

Author: vistart

src/rhosocial/activerecord/backend/impl/sqlserver/functions/string.py

@@ -5,7 +5,7 @@
 Functions: string_agg, concat_ws, trim, format
 """
 
-from typing import Union, Any, TYPE_CHECKING, List
+from typing import Union, Any, TYPE_CHECKING, List, Optional
 
 from rhosocial.activerecord.backend.expression import bases, core